Optical detectors are critical components in optical and magneto-optical data storage systems. Historically, amplifiers for photosensitive diodes used in optical detectors have been of the transimpedance type and have had a single-ended rather than a differential input. A transimpedance amplifier has been used because the signal generated by the photosensitive diode has been in the form of a current when the diode is reverse-biased with a substantial voltage (typically several volts or greater), and the desired output has been in the form of a voltage. It has also been convenient to tie the cathode of the diode to the positive supply voltage so that a typical operational amplifier could be used with resistor feedback to effect a transimpedance gain. This configuration results in half of the available signal current from the diode being effectively dumped into the positive supply, causing a signal reduction at the input to the preamplifier. Moreover, this kind of configuration has no common-mode rejection for injected common-mode currents (caused by, e.g., parasitic capacitances to the environment). Any common mode current passes through as though it were a signal current; and the common mode rejection ratio (CMRR) is 0 dB.
The only prior art of interest of which applicants are aware in Japanese application 63-249382 published Oct. 17, 1988. That application does not provide details of the amplifier construction. However, it shows a photodiode biased indirectly by impedances (resistors) between a power supply and the photodiode, which is alleged to eliminate the common-mode signal. The photodiode is decoupled from the power supply using capacitors. This would imply a high input impedance. Capacitive coupling between the photodiode and amplifier would imply a voltage amplifier, although it is not clear whether the amplifier is a differential voltage or differential current type. The circuit cannot offset power supply noise because the photodiode is biased through resistors of limited size. Any attempt to increase gain by increasing the values of the patentee's resistors (3 and 4) would increase the signal voltage level (at B and C), but degrade the frequency response and hence bandwidth and provide parasitic capacitance between B, C and ground.
There is a need for an optical data storage system comprising an amplifier which:
(i) provides a differential current input having a low impedance over the full range of frequencies for which amplification is required in an optical detector that includes a photodiode that senses the intensity of an optical signal generated by data on an optical storage medium; PA1 (ii) reverse biases the photodiode; PA1 (iii) amplifies the signal current from both sides of the photodiode to improve signal-to-noise ratio; and PA1 (iv) is isolated from and unaffected by power supply noise.